These describe the requirements for plain x-rays within the clinical context of an ankle injury. They state that:

an ankle radiograph is required only if there is pain in the "malleolar zone" and any of these findings:

Bone tenderness at the posterior edge/tip of lateral malleolus (6cm).
Bone tenderness at the posterior edge/tip of medial malleolus (6cm).
Inability to weight bear, both immediately and in the casualty department.

a foot radiograph is required only if there is pain in the "midfoot zone" and any of these findings:

Bone tenderness at the base of the 5th metatarsal.
Bone tenderness at the navicular.
Inability to weight bear, both immediately and in the casualty department.

Projections

AP mortise
This is obtained with internal rotation so that the fibula does not overlap the talus. This is important so that any joint space widening can be demonstrated. It also serves to clear the lateral joint space so that lateral talar dome fractures can be identified. Assess for any soft tissue swelling medially and laterally.

Lateral
Open collimation to include the calcaneum, 5th metatarsal base and dorsal surfaces of the talus and navicular (all common sites for avulsion fractures). Look for an anterior joint effusion, as this can be significant. An effusion is seen as an area of increased density that pushes out the adjacent fat plane.

Suboptimal positioning AP mortise
?Talar dome fracture. However, appearance is due to inadequate mortise view, and the lateral joint space not being clearly demonstrated:

Suboptimal positioning lateral ?Fracture lateral malleolus. However, appearance is due to inadequate lateral view and the medial and lateral malleoli not being superimposed:

Accessory ossicles - Normal variants

Unfused secondary ossification centres. These are well corticated osseous fragments that are located in characteristic places. Should not be misinterpreted as fractures; refer to Keats Atlas of Normal Variants if unsure. Some of the more common ossicles are:

Os subfibulare - distal to the tip of the lateral malleolus.
Os subtibiale - distal to the tip of the medial malleolus.
Os trigonum - posterior to the talus.

Mortise fractures

As the ankle is a ring structure, if one displaced fracture is seen, another should be sought. The second injury may not be a fracture, but a ruptured ligament, which will be evident by widening of the joint. The main ligaments binding the ankle are:

the medial and lateral collateral ligaments. These may rupture with or without an attached bony fragment:

the interosseous ligament. The interosseous ligament may be assessed by ensuring that at approximately 1cm proximal to the tibial plafond, the distance between the tibia and fibula is not greater than 6mm. If greater than 6mm, a rupture should be suspected:

Full length tib & fib views are necessary if an apparently isolated fracture is seen at

the medial malleolus (especially if displaced).

the posterior malleolus (especially if displaced)

or isolated widening of the medial joint space.

The second injury may be a fracture at the proximal fibula. This is referred to as a Maisonneuve injury:

If an apparently isolated fracture of the distal fibula is identified on the lateral view, search for a further fracture of the posterior malleolus which may be superimposed over the fibula fracture:

Weber Classification

A =

Lateral malleolar fracture below the level of the syndesmosis.

Medial malleolar fracture; oblique (if present).

B =

Lateral malleolar fracture at the level of the syndesmosis, running proximally.

Medial malleolar fracture; transverse. Or a tear of the ligament.

C =

Lateral malleolar fracture originating proximal to the joint line.

Medial malleolar fracture; transverse. Or a tear of the ligament.

Additional to B or C may be a posterior malleolus fracture.

Salter-Harris Classification

Refers to fractures within the paediatric skeleton which involve the physeal growth plate. An injury to this area will cause a fracture to the weakest point, which is the cartilaginous growth plate, and spare the stronger joint capsule, ligaments and tendons. Depending on the involvement of the adjacent metaphysis and epiphysis, determines the classification:

Salter-Harris Type I = Injury through the physeal growth plate only, usually with displacement. No involvement of the adjacent metaphysis or epiphysis:

Salter-Harris Type III (Tillaux fracture) = Fracture line running through the tibial growth plate and adjacent epiphysis. This is intra-articular, and occurs at the site of ligamentous insertion (anterior tibio-fibular ligament). The distal tibial growth plate fuses from medial to lateral. The Tillaux fracture occurs in older children at the antero-lateral tibial epiphysis before complete fusion has occurred:

Salter-Harris Type IV (triplane fracture) = The triplane fracture involves the epiphysis, physeal plate and metaphysis. The fracture runs through the tibial epiphysis in the sagittal plane, horizontally through the physeal growth plate and through the posterior tibial metphysis in the coronal plane:

It is important to detect fractures through the physeal growth plate as, if missed and not treated, the growth plate may fuse prematurely causing deformity. As the classification increases from I to V, the frequency of injury decreases (type I and II common) but the severity of injury increases.

Toddler's Fracture

Refers to a spiral fracture of the tibia. This is often undisplaced and is therefore difficult to detect; usually is only demonstrated on one projection. Occurs when the child (1-3years) twists and falls, with one leg fixed on the floor.

Talus

The talar dome should be scrutinised for a subtle indentation of the joint surface, or a small detached fragment. This is evidence of an osteochondral fracture. May be subtle, is often missed, but this injury is clinically significant:

Osteochondritis dissecans is characteristically seen within the talus. Osteochondritis dissecans refers to a subarticular lucency with sclerotic margins, within which is seen an oval segment of bone (area of avascular necrosis). This will often give symptoms similar to the presence of a loose body, as there becomes a gradual separation of loose bone.

Avulsion off the dorsal surface of the head of the talus is common, as is the dorsal surface of the navicular, both identified on the lateral:

True sub-talar dislocation consists of dislocation of the talo-navicular and talo-calcaneum joints with navicular and calcaneum displaced with the foot. Frequently accompanied by fractures of the head/neck/dome of talus:

Calcaneum

Non-compressive avulsion fractures are common, particularly off the anterior process of the calcaneum, where the bifurcate ligament inserts. This may be identified on the lateral ankle or oblique foot projections:

A second common avulsion site is seen laterally on either the AP ankle or DP foot, at the insertion of the extensor digitorum brevis muscle:

Compressive fractures occur after a fall from a height. Subtle fractures may only be identified by assessing Boehler's angle. This angle is measured by drawing a line from the highest point of the posterior tuberosity to the highest midpoint, and a 2nd line from the highest midpoint to the highest point of the anterior process. The angle, posteriorly, should be >30 degrees. If there is flattening of the bone due to a fracture, this angle will be decreased, to <30 degrees. Also assess the trabecular pattern for evidence if impaction (sclerosis):